The present invention relates to a system, particularly suitable for use in ink-jet printing apparatus, wherein image data of an original resolution is output by the printing apparatus at an enhanced resolution higher than the original resolution.
In digital printing of images based on original image data, the overall performance of the printing system is constrained by a number of factors. Among these factors are the processing speed by which image data is caused to activate the printing hardware, the desired resolution of the output image, and the applicability of image-enhancement techniques such as the use of error diffusion to simulate grayscale or halftone images. Outputting an image at a higher resolution is generally always desirable, but processing of enhanced image data consumes processing time, and image-enhancement techniques, such as error diffusion to simulate halftones and grayscales, may not always be compatible with resolution enhancement techniques.
Ink-jet printing, whether based on thermal ink jet technology or piezoelectric or acoustic ink jet technology, is well known. As shown in FIG. 1, regardless of the basic ink ejection technology, a typical arrangement of an ink-jet printer includes a printhead PH of relatively small dimensions which moves in a fast scan direction FS across a sheet S, thus forming a band B of printed area as the ejectors in the printhead propel ink onto the sheet S. Because of the relatively small size of printhead PH, the width of any band B will be small relative to the sheet S, and thus, as is known in the art, it is common to have sheet S be indexed in the index direction as shown, so that a series of swaths formed by a succession of bands B make every portion of a sheet S available to the printhead PH.
One generally-known technique for improving the overall image quality of an ink-jet apparatus, whether monochrome or color, is to permit the printhead PH to print on any particular area on sheet S multiple times during the printing process. One simple way in which this can be accomplished is to have a xe2x80x9ctwo-passxe2x80x9d system, wherein, for each band B, the printhead PH moves first in one direction along fast scan direction FS, and then returns in the opposite direction along the fast scan direction FS before the paper is indexed to print the next band. In this way, the printhead PH gets two opportunities for printing in the area of a particular band B. Systems which permit more than two passes of the printhead over any particular area, such as a four-pass system, can readily be contemplated.
Another technique for improving the overall image quality, particularly in ink-jet printing apparatus, is to increase the ejection rate of the printhead as it moves relative to the sheet, thereby increasing the output resolution of the system at least along the fast scan direction FS. Typical in the marketplace are inexpensive ink-jet printing apparatus with double resolution along with fast scan direction, such as an output resolution of 600xc3x971200 spi, which operate in this way. (With ink-jet printing apparatus, to increase the output resolution in the other dimension would either require replacement of the printhead with a printhead with more ejectors per unit length or making advances of the media include movements that are not integral multiples of the ejector spacing.)
The present invention is directed to a method of operating a digital printer which improves the performance of the system, particularly in regard to the techniques of multiple-pass printing and resolution enhancement.
According to one aspect of the present invention, there is provided a method of printing digital image data with a digital printer. The image data has print-black and print-white pixels arranged in an original resolution, and the digital printer is capable of outputting an output image having an output resolution that is higher in at least one dimension than the original resolution. For each pixel in the image data, the pixel is associated with a set of subpixel areas, the subpixel areas for each pixel being arranged in positions to reflect a relationship of the original resolution to the output resolution. For each print-black pixel in the image data, a print-black datum is assigned to a subpixel area. The locations of print-black data among subpixel areas are distributed whereby, for a set of print-black pixels, the positions of the print-black data are substantially evenly distributed among positions of subpixel areas.